1. Field of the Invention
The present invention generally concerns shipboard design to combat Aquatic Nuisance Species (ANS) invasion resulting from ballast water discharge.
The present invention particularly concerns ballast water treatment, deoxygenation and carbonation of ballast water, reduction of pH in ballast water, infusion of inert gas into ballast water, control of aquatic nuisance species, bubbling of inert gas through and into ballast water, and elevated CO2 levels in ballast water.
2. Background of the Invention
2.1 Aquatic Nuisance Species Present in Ship""s Ballast Water
It is estimated that 21 billion gallons of ballast taken on in foreign ports are discharged by commercial vessels annually in the waters of the United States (Carlton et al. 1993). Ballast water transport is a major vector for introduction of potentially invasive aquatic species.
Standards for treatment of ballast water are still in a state of flux. Efforts to define standards are ongoing in the US Congress, International Maritime Organization (IMO), and other individual maritime nations. The US Congress (NAISA 2002) proposes an Act that will, among other considerations, set the interim standards for ballast water treatment (BWT). It states, xe2x80x9cThe interim standard for BWT shall be a biological effectiveness of 95% reduction in aquatic vertebrates, invertebrates, phytoplankton and macroalgae.xe2x80x9d There are discussions about setting micron standards, i.e. x microns cut-off for living organisms. Currently, a fifty (50) micron standard is being discussed in various circles, including IMO and US Coast Guard. The default standard appears to be the Ballast Water Exchange (BWE), or something close to it. Cangelosi (2002) states xe2x80x9c . . . the Coast Guard has set forth a xe2x80x9cdo-it-yourselfxe2x80x9d approach, directing interested ship owners to conduct complex shipboard experiments (post-installation) to undertake direct and real-time comparisons between BWE and treatment. If the comparison is favorable and defensible, the Coast Guard will approve the treatment. See Cangelosi, Allegra (Nov. 14, 2002). Testimony Before the Joint Committee on Resources and Science of the U.S. House of Representatives.
2.1 Control of Aquatic Nuisance Species Present in Ship""s Ballast Water
Glosten (2002) provides a review of the numerous treatment systems for the control of aquatic nuisance species in ship""s ballast water. These systems include heat, cyclonic separation, filtration, chemical biocides, ultraviolet light radiation, ultrasound, and magnetic/electric field. See Glosten-Herbert-Hyde Marine (April, 2002). xe2x80x9cFull-Scale Design Studies of Ballast Water Treatment Systemsxe2x80x9d, Prepared for Great Lakes Ballast Technology Demonstration Project.
Known methods not mentioned in this reference are hypoxia, carbonation, and their combination. In studies of 18 months duration on a coal/ore vessel (Tamburri et al. 2002), the ballast water dissolved O2 level was reduced and held to concentrations at or below 0.8 mg/l by bubbling essentially pure nitrogen. See Tamburri, M. N., Wasson K., and Matsuda, M. (2002). Ballast water deoxygenation can prevent aquatic introductions while reducing ship corrosion. Biological Conservation. 103, 331-341. The experiments resulted in a treatment xe2x80x9cthat can dramatically reduce the survivorship of most organisms found in the ballast water . . . xe2x80x9d
In extensive experiments with gas of varying percent CO2, N2 and O2 (McMahon, et al. 1995), the xe2x80x9c . . . results indicate that CO2 injection may be an easily applied, cost-effective, environmentally acceptable molluscicide for mitigation and control a raw water system macrofouling by Asian clams and zebra musselsxe2x80x9d. See McMahon, R. F., Matthews, M. A., Shaffer, L. R. and Johnson, P. D. (1995). Effects of elevated carbon dioxide concentrations on survivorship in zebra mussels (Dreissena polymorpha) and Asian clams (Corbicula fluminea). In The fifth international zebra mussel and other aquatic nuisance organisms conference, pp. 319-336. Toronto, Canada.
2.3 Corrosion Considerations of Various Ballast Water Treatment Systems
Shipboard corrosion mitigation is always a priority consideration. It requires the continual attention of the crew and, if not carefully controlled, can actually compromise the strength of the ship. Any installed ballast water treatment system must not under any circumstances increase the potential for corrosion, and if possible, should decrease the potential. The present invention will be seen to have considered the corrosion issue.
As reported in literature Tamburri et al. (2002), corrosion might even be mitigated by deoxygenation. See Tamburri, M. N., Wasson K., and Matsuda, M. (2002), op cit.
Perry, et al. (1984) state that unless pH level drops below 4 concerns about corrosion are unfounded. See Perry, R. H., Green, D. W., Maloney, G. O. Perry""s Chemical Engineer""s Handbook, 5th Ed., McGraw Hill, 1984.
2.4 The Theory of Ballast Water Treatment by Anoxia and/or Hypoxia
Except for ballast water exchange, essentially all treatment concepts involve the chemical change of the water to cause an environment lethal for ANS. The chemical changes described in Tamburri et al. (2002) and McMahon (1995) offer promising results, i.e., reduce the dissolved O2 in the one case, and carbonate and reduce the pH in the other case. See Tamburri, M. N., Wasson K., and Matsuda, M. (2002), op cit. See also McMahon, R. F., Matthews, M. A., Shaffer, L. R. and Johnson, P. D. (1995), op cit.
In both cases the process involves the exchange of gases, the extraction of the dissolved O2 and the introduction of CO2. Surface contact area and partial pressure differentials permit the gas exchanges to occur. The deoxygenation of the ballast water is based on Henry""s Law of gas solubility: The relative proportion of any dissolved gas including oxygen in the ballast water is a function of the concentration, equivalent to partial pressure of the gas (e.g. oxygen), within the mixed gases over the ballast water. The depletion of oxygen in the ballast water is primarily a function of the shared surfaces and concentrations at the interfaces of the inert gases and water.
The pH of the ballast water is lowered by the chemical reaction:
CO2+H2Oxe2x86x92H2CO3⇄H++HCO3xe2x88x92
This equation is interpreted that carbon dioxide (CO2) reacts with water (H2O) to form carbonic acid (H2CO3), which then partially dissociates to form hydrogen (H+) and bicarbonate ions (HCO3xe2x88x92).
All systems described thus far in the literature, including ballast transfer, have left untreated the sediment buildup in the bottom of the tanks. If the orifices in the lattice work of piping were to point down, then the sediment could potentially be stirred up, facilitating the killing of the embedded ANS.
2.2 Ballast Water Treatment in the Related Predecessor Patent Application
The user of gaseous underpressure in the treatment of ship""s ballast water so as to combat Aquatic Nuisance Species (ANS) invasion resulting from ballast water discharge, described in this application, is an extension of American Underpressure System (AUPS) of MH Systems, San Diego, Calif. The AUPS utilizes a slight negative pressure in the tank""s ullage space, in an inert environment, to prevent or minimize oil spillage from tankers (Husain et al. 2001). See Husain, M., Apple, R., Thompson, G. and Sharpe, R. (2001); Full Scale Test, American Underpressure System (AUPS) on USNS Shoshone, presented to Northern California Section, SNAME, September 2001.
The American Underpressure System (AUPS) is the subject of U.S. Pat. No. 5,156,109 for a System to reduce spillage of oil due to rupture of ship""s tank, and U.S. Pat. No. 5,092,259 for Inert gas control in a system to reduce spillage of oil due to rupture of ship""s tank. It is also the subject of related U.S. Pat. No. 5,343,822 for Emergency transfer of oil from a ruptured ship""s tank to a receiving vessel or container, particularly during the maintenance of an underpressure in the tank; U.S. Pat. No. 5,323,724 for a Closed vapor control system for the ullage spaces of an oil tanker, including during a continuous maintenance of an ullage space underpressure; and U.S. Pat. No. 5,285,745 for System to reduce spillage of oil due to rupture of the tanks of unmanned barges. All patents are to the selfsame inventor Mo Husain who is one of the co-inventors of the present invention.
The AUPS is retrofittable on existing tankers, and has the similar spill avoidance capability as that of a double hull tanker during accidental rupture of the hull. The AUPS spill avoidance system creates a slight vacuum (two to four pounds per square inch) in each cargo tank. This vacuum, assisted by the outside hydrostatic pressure of the surrounding water, prevents or minimizes cargo loss in the event of hull rupture. In case of a bottom rupture caused by grounding, nearly all of the cargo can be protected. In the case of side hull damage, cargo below the level of the damage will be lost, while the cargo above the side hull rupture will be protected.
This system can be used in conjunction with existing inert gas systems that are mandatory on most tankers to prevent explosions. The AUPS consists essentially of exhaust blowers with their isolation and control valves tapping into the inert gas system. A negative pressure of inert gas is created in the ullage spacexe2x80x94the volume of gas above the oil. This negative pressure or underpressure is continuously adjusted and prevents oil from spilling if the tanker is ruptured. Stated simply, the oil is held in the tank by the slight underpressure.
This partial vacuum, or underpressure, assisted by the outside hydrostatic pressure of the surrounding water, prevents or minimizes cargo loss in the event of hull rupture. In case of a bottom rupture caused by grounding, nearly all of the cargo can be protected. In the case of side hull damage, cargo below the level of the damage will be lost, while the cargo above the side hull rupture will be protected.
This negative pressure or underpressure is continuously adjusted and prevents oil from spilling if the tanker is ruptured. Again stated simply, the oil is held in the tank by the slight underpressure.
As of 2003, the environmental threat posed by oil tanker accidents has mandated the use of double-hull construction. However, the phase-out of conventional xe2x80x9csingle-skinxe2x80x9d tankers may last to 2015. One goal of the AUPS system, including as is modified and enhanced by the present invention, has been and remains, circa 2003, to provide the protection until all existing single-skin tankers visiting U.S. ports are retired.
The present patent application is also related as a Continuation-in-Part to U.S. patent application Ser. No. 10/120,339 filed on May 9, 2002, for CLOSED LOOP CONTROL OF BOTH PRESSURE AND CONTENT OF BALLAST WATER TANK GASES TO AT DIFFERENT TIMES KILL BOTH AEROBIC AND ANAEROBIC ORGANISMS WITHIN BALLAST WATER to inventor Henry Hunter assigning to the same MH Systems, San Diego, Calif., that is the assignee of the present invention. That application is itself a Continuation-In-Part (C-I-P) of U.S. patent application Ser. No. 09/865,414 filed May 25, 2001, for CLOSED LOOP CONTROL OF VOLATILE ORGANIC COMPOUND EMISSIONS FROM THE TANKS OF OIL TANKERS, INCLUDING AS MAY BE SIMULTANEOUSLY SAFEGUARDED FROM SPILLAGE OF OIL BY AN UNDERPRESSURE SYSTEM, now issued as U.S. Pat. No. A,AAA,AAA.
As a simplified basis of comparison, the first related predecessor application may be considered to teach the control of oxygen in ship""s ballast water maintained under a pressure less than atmosphere for the inducement, at different times, of both such (i) oxygen-starved and (ii) oxygen-rich conditions as are respectively fatal (i) to aerobic marine organisms (by action of hypoxia), and (ii) to anaerobic marine organisms (by action of exposure to high levels of dissolved oxygen).
Meanwhile, the present application will be seen to teach the inducement of each of (i) carbon dioxide-rich, (ii) acid-enhanced and/or (iii) oxygen-starved conditions in ship""s ballast waterxe2x80x94preferably as is continuously maintained under a pressure less than atmosphere pressurexe2x80x94so as to induce, at one and the same time, (i) hypercapnic, (ii) acidic and/or (iii) hypoxic conditions that are fatal to both aerobic, and anaerobic, marine organisms.
The present invention contemplates the infusion of inert, or combustion, gases into ballast waterxe2x80x94preferably as is maintained under less than atmospheric pressurexe2x80x94in order to kill harmful aquatic nuisance species by simultaneous, synergistic, inducement of (1) hypercapnia (elevated concentration of dissolved CO2), (2) hypoxia (depressed concentration of dissolved O2), and (3) acidic pH level. The inert combustion gases may be obtained, for example, from (i) a ship""s inert gas generator (of the Holec, or equivalent types), and/or from (ii) ship""s own flue gases. These gases are highly noxious, having CO2 significantly increased and O2 significantly depleted, from normal atmospheric levels. An air-breathing animalxe2x80x94not only humans, but lower animalsxe2x80x94would soon be stifled by these gases. Thus one way to think about the prophylactic action of present invention is to consider that the present invention effectively and efficiently alters the mixture of atmospheric gases, including oxygen (O2), that normally are dissolved in ballast water in favor of, predominantly, carbon dioxide (CO2). Aquatic marine organismsxe2x80x94at least of the aerobic typesxe2x80x94can scarcely tolerate these noxious gases any better than can air-breathing animals, and a widespread and severe die-off of multiple marine organisms, is experienced in the presence of these noxious gases dissolved in sea water.
1. The Present Invention Starts With Inducing (1) Hypercapnia, and, in Association with Elevated CO2, (2) Depressed pH
The present invention contemplates the control of Aquatic Nuisance Species (ANS) present in the ballast water of ship""s ballast tanks by action of inducing hypercapnia (fatally elevated CO2 levels) in marine organisms present within the ballast water. The same elevated CO2 levels as induce hypercapnia also serve to acidify the sea water.
This condition of enhanced dissolved CO2xe2x80x94which is of an extreme level such as strongly induces hypercapnia in marine organismsxe2x80x94is, in accordance with the present invention, preferably realized by infusion of a mixture gases into the seawater, which gaseous mixture is preferably enhanced in CO2 to xe2x89xa711% by molar volume and, more preferably, to xe2x89xa715% by molar volume. In accordance with the invention, these gases enhanced in CO2 are preferably realized as the gaseous output of a standard shipboard inert gas generator (commonly called a Holec generator, after the major manufacturer thereof) (which output is commonly about 84% Nitrogen, 12-14% CO2 and 2% Oxygen), and/or as a ship""s own flue gases. These preferred CO2 concentrations may be compared with, by way of example, published studies of hypercapnia in marine organisms that have generally investigated introduction of gaseous mixtures having CO2 concentrations in the range from 0.1% to 1%. In accordance with the present invention, effective delivery of the gases high in CO2 concentration into ballast water will be realized by bubbling these gases into a ballast water from the bottom of a ballast water tank that is maintained at pressure less than atmosphere (called an xe2x80x9cunderpressurexe2x80x9d in this and in related patent applications)xe2x80x94but this aspect of the invention will be further dealt with later.
The infusion of the gases enhanced in percentage CO2 is preferably continued until dissolved CO2 in the ballast water is raised to xe2x89xa720 ppm, and more preferably to xe2x89xa750 ppm.
Dissolved CO2 of this level serves to acidify sea water. The chemical mechanism by which enhanced dissolved CO2 acidifies seawater is well established, and is:
CO2+H2Oxe2x86x92H2CO3⇄H++HCO3xe2x88x92
Dissolved CO2 of the preferred levels of xe2x89xa720 ppm reduces the pH of seawater, which is normally 8, to acidic levels of pHxe2x89xa67, and, preferably, pHxe2x89xa66 and still more preferably pHxe2x89xa65.5.
It is hard to tell whether the dissolved CO2 at concentrations xe2x89xa720 ppm, or the acidic levels of pHxe2x89xa67, are more injurious to the ANSxe2x80x94being that both are relatedxe2x80x94but research indicates that both factors are individually effective in killing ANS, and both factors together appear to be usefully synergistic in killing ANS.
2. The Present Invention Continues With Inducing (3) Hypoxia in Aquatic Nuisance Species Present in Ballast Water
Still further, the present invention contemplates not to stop with simply inducing conditions in ballast water that are both hypercapnic and acidic to ANSxe2x80x94injurious and fatal to ANS as these conditions alone may bexe2x80x94but to continue by depriving these ANS of oxygen at the same time. In particular, this extension and enhancement of the present invention is based on the recognition that (i) aquatic nuisance species present in ship""s ballast water may best be controlled by a combination of hypoxic, hypercapnic and acidic conditions within the ballast water, and that (ii) these conditions may be simultaneously economically realized by bubbling gases from an inert gas generator, and/or the flue gases of the ship, through the ballast water, preferably as the ballast water is maintained under a pressure less than atmosphere. The preferred levels of dissolved CO2 (i.e., preferably xe2x89xa720 ppm, and more preferably to xe2x89xa750 ppm), and the preferred pH levels (i.e., to pHxe2x89xa67, and, preferably, pHxe2x89xa66 and still more preferably pHxe2x89xa65.5), have already been stated. In accordance with the present invention, the oxygen content of a gaseous mixture that infused with ballast water is preferably xe2x89xa64% O2, and is more preferably xe2x89xa63% O2, and this infusion of is continued until a dissolved oxygen level of, preferably, xe2x89xa61 ppm O2 and, more preferably, xe2x89xa60.8 ppm O2 is induced.
Importantly to understanding the present invention, it should be appreciated that the most preferred method of the invention is managing at least three different conditionsxe2x80x94each of two dissolved gases, and acidity/alkalinityxe2x80x94all at the same time.
To appreciate that the conditions are separate, and separately managed, understand to begin with that hypoxia, or lack of oxygen, implies neither hypercapniaxe2x80x94an excess of carbon dioxidexe2x80x94nor acidityxe2x80x94a pH less than seven. For example, oxygen present in ullage space gases and/or as a dissolved gas in ballast water may be replaced with nitrogen without appreciable effect on either (i) the dissolved carbon dioxide within, or (ii) the pH balance of, the ballast water.
Likewise, it should be understood that hypercapnia, or an excess of carbon dioxide, does not mandate hypoxia, nor an acidic pH. For example, the carbon dioxide level in the enclosed atmosphere of a submarine can, as a product of human respiration, rise to high levels but that it is xe2x80x9cscrubbedxe2x80x9d from the atmosphere. The build-up of CO2 can transpire in an enclosed space nonetheless that the atmosphere may constantly contain copious oxygen (derived on a nuclear submarine from the electrolysis of water with electricity).
Finally, even when carbon dioxide is added to waterxe2x80x94as it sometimes is by aquarists to promote the lush growth of aquatic plantsxe2x80x94this augmentation of dissolved CO2 gas need not result in decreased pH (increased acidity) of the water (by the same chemical mechanism as occurs in the present invention) if, as is often the case, any lowering of the pH level is counteracted by the addition of a chemical base such as, most commonly, lime.
Accordingly, even though the three conditions of (1) hypoxia, (2) hypercapnia and (3) reduced pH, or acidity, will be seen to be relatively straightforwardly realized by the preferred methods and system of the present invention by the addition of but a single mixture of gases all at the same time, these three conditions within ballast (or other waters) are not simply happenstantially achieved, but are instead, in accordance with the teaching of the present invention, intentionally realized.
3. The Present Invention Realizes Gaseous Exchange in Ballast Water Efficiently, and Effectively
Importantly to economically, and practically, realizing the most preferredxe2x80x94ANS-killingxe2x80x94conditions within a ship""s ballast water, the preferred ballast water treatment method in accordance with the present invention consists of (i) bubbling an oxygen-depleted, CO2-enhanced, inert gas mixture via a row of pipes (orifices at the bottom of the pipes) located at the bottom of a ballast water tank, while (ii) maintaining a negative pressures of xe2x88x922 psi at the ullage space of the same ballast water tank.
As explained in the first related predecessor patent application, the bubbling at, and during, an underpressure in the ballast water tanks makes that (some) exchange of dissolved gases is realized by (i) outgassing as transpires over the huge combined surface area of the bubbles, as opposed to (ii) mere slow diffusion of dissolved gases through the ballast water, with gaseous interchange occurring essentially only at the surface layer of the tank.
The inert gas is preferably from a standard shipboard inert gas generator (commonly called a Holec generator), and is commonly composed of about 84% Nitrogen, 12-14% CO2 and 2%-4% Oxygen. In accordance with the present invention, the ballast water is equilibrated with gases from the inert gas generator. As a result, the water will become hypoxia, will contain CO2 levels much higher than normal, and the pH will drop from the normal pH of seawater (pH 8) to approximately pH 6.
Ballast water treatment in accordance with the present invention has undergone preliminary laboratory tests at the Scripps Institution of Oceanography, La Jolla, Calif. USA, and has realized the results reported in this specification.
4. A Method of Killing Aquatic Nuisance Species in Ship""s Ballast Water by Hypercapnia, or Combined Hypercapnia and Hypoxia
Therefore, in one of its aspects the present invention is embodied in a method of killing aquatic nuisance species in ship""s ballast water. The base method consists simply of infusing carbon dioxide into the ship""s ballast water at a level effective to kill aquatic nuisance species by hypercapnia.
The infusing is preferably with a gaseous mixture of xe2x89xa711% carbon dioxide by molar volume. This infusing with the gaseous mixture of xe2x89xa711% carbon dioxide preferably transpires until the ballast water is hypercapnic to xe2x89xa75 ppm dissolved carbon dioxide. This infusing preferably transpires by bubbling the gaseous mixture through the ballast water, and more preferably by bubbling of the gaseous mixture is through the ballast water that is under less than atmospheric pressure. In particular, the ballast water under less than atmospheric pressure is preferably located within ballast water tanks of the ship where ullage space gas pressure is xe2x88x922 p.s.i. below atmospheric pressure, or lower.
The base method is preferably expanded, or enlarged, to include concurrently depleting oxygen in the ship""s ballast water at a level effective to kill aquatic nuisance species by hypoxia.
In this expanded method the infusing is preferably like as in the base method, with the depleting preferably transpiring by substitution of gases, including oxygen gas dissolved in the ballast water, with a gaseous mixture of xe2x89xa64% oxygen. This depleting with a gaseous mixture of xe2x89xa64% oxygen preferably transpires until the ballast water is hypoxic to xe2x89xa61% ppm dissolved oxygen.
As with the infusing, the depleting transpires by bubbling the gaseous mixture through the ballast water. This bubbling of the gaseous mixture is again through the ballast water that is under less than atmospheric pressure, and is more preferably through ballast water within ballast water tanks of the ship where tank ullage space gas pressure is xe2x88x922 p.s.i. below atmospheric pressure, or lower.
In either the base, or the expanded, method, the infusing and/or the depleting may be, and preferably is, accompanied by acidifying of the ship""s ballast water at a level effective to kill aquatic nuisance species.
This acidifying is a consequence of the infusing where, as is preferred, the infusing is with a gaseous mixture of xe2x89xa611% carbon dioxide by molar volume. In this case the acidifying is then concurrently realized by the chemical reaction
CO2+H2Oxe2x86x92H2CO3⇄H++HCO3xe2x88x92.
More particularly, the infusing with the gaseous mixture of xe2x89xa711% carbon dioxide preferably transpires until both (1) the ballast water is hypercapnic to xe2x89xa720 ppm carbon dioxide, and (2) the same ballast water is acidic to pHxe2x89xa67.
As before, the infusing and, consequent to the infusing, the acidifying preferably transpires by bubbling the gaseous mixture through the ballast water, and more preferably through the ballast water that is under less than atmospheric pressure, most preferably xe2x88x922 p.s.i. below atmospheric pressure, or lower.
Likewise that the infusing (of CO2) preferably transpires the same in the basis, and in the extended, methods, so also does the depleting (of O2) preferably transpire the same even when the consequence of the depleting is measured in the acidification, or the lowering of the pH of the ballast water, instead of, or in addition to, the inducing of hypercapnic and/or hypoxia conditions.
Further likewise, the depleting (of CO2) and/or the depleting (of O2) preferably transpires by the same bubbling process, most preferably into ballast water at less than atmospheric pressure, when the consequence of the depleting is measured in the acidification, or the lowering of the pH of the ballast water, instead of, or in addition to, the inducing of hypocapnic and/or hypoxia conditions.
In simple terms, the process steps of the present invention are consistent, and synergistic. Everything works together, in concert and to the same end: the killing of aquatic nuisance species in ship""s ballast water.
5. A Quantitative Method of Reducing Survival of Aquatic Nuisance Species in Ship""s Ballast Water
In another of its aspects the present invention may be considered to be embodied in a quantitative method of reducing survival of aquatic nuisance species in ship""s ballast water that is, in the preferred parameters of its conduct, quite unlike any prior art with which the inventors are acquainted. In simple terms, the method of the present invention renders ballast water triply deadly to aquatic nuisance species due to each of hypoxia, hypercapnic and acidic conditions.
In the preferred method a gaseous mixture consisting essentially of xe2x89xa780% nitrogen, xe2x89xa711% carbon dioxide and xe2x89xa64% oxygen through ship""s ballast water until the ballast water is permeated to equilibrium with these gases, at which time the ballast water will be hypoxia to xe2x89xa61 ppm oxygen, hypercapnic to a xe2x89xa720 ppm carbon dioxide, and acidic to pHxe2x89xa67.
The permeated gaseous mixture is preferably the output of a marine inert gas generator. This gaseous mixture that is output from a marine inert gas generator consists essentially of nitrogen in the range from 87% to 84% mole percent, carbon dioxide in the range from 14% to 11% mole percent, and oxygen in the range from 2% to 4% mole percent.
Regardless of the particular ratios of the gaseous components of the gaseous mixture, the permeation is most preferably continued until the ship""s ballast water until the ballast water is hypoxic to xe2x89xa60.8 ppm oxygen, hypercapnic to xe2x89xa750 ppm carbon dioxide, and acidic to pHxe2x89xa66.
As will by now be familiar, the gaseous mixture is preferably permeated to equilibrium within the ballast water by being bubbled through the ballast water, and more preferably through ballast water that is at a pressure less than atmosphere.
6. A System for Reducing Survival of Aquatic Nuisance Species in Ship""s Ballast Water
In yet another of its aspects, the present invention is embodied in a system for reducing survival of aquatic nuisance species in ship""s ballast water.
The preferred system includes (1) a gas generator producing a gaseous mixture enhanced in carbon dioxide relative to both (i) atmospheric proportion of carbon dioxide, and (ii) proportion of carbon dioxide that is dissolved in sea water, (2) piping having and defining discharge orifices at the base of, and inside, the ship""s ballast water tank; and (3) a compressor pressuring the gaseous mixture received from the gas generator sufficiently so that, as delivered to the piping, it will be forced out the discharge orifices and bubble upward through the ballast water.
In this system gaseous interchange transpires between (i) the gaseous mixture, enhanced in carbon dioxide, that is within the bubbles and (ii) dissolved gases within the ballast water. This gaseous interchange transpires until dissolved gases within the ballast water will become enhanced in carbon dioxide to a level inducing hypercapnia in aquatic nuisance species within the ballast water.
In this basic system the gas generator preferably produces a gaseous mixture having xe2x89xa711% carbon dioxide by molar volume.
This basic system is preferably expanded and enhanced by causing that the same gas generator producing the gaseous mixture enhanced in carbon dioxide also produces the gaseous mixture that is concurrently diminished in oxygen over both (i) atmospheric proportion of oxygen, and (ii) proportion of oxygen dissolved in sea water. The gas generator is thus called an xe2x80x9cinertxe2x80x9d gas generator.
In this expanded, and enhanced, system the gaseous interchange transpiring between (i) the gaseous mixture, diminished in oxygen, that is within the bubbles and (ii) the dissolved gases within the ballast water, causes dissolved gases within the ballast water to become diminished in oxygen to a level inducing hypoxia in aquatic nuisance species within the ballast water.
The inert gas generator preferably produces a gaseous mixture having xe2x89xa711% carbon dioxide by molar volume, and, most preferably, xe2x89xa64% oxygen by molar volume.
In either the basic, or the expanded and enhanced, systems a blower preferably evacuates gases from within the ullage space of the ship""s tank so as to produce a pressure therein which is at least 2 p.s.i. less than prevailing atmospheric pressure outside the tank.
The piping preferably includes a matrix of piping in a grid array at the base of, and inside, the ship""s ballast water tank. Discharge orifices of this piping are variously directed both upwards toward the top and the tank and downwards towards the base of the tank.
The compressor preferably produces a pressure more than 2 p.s.i. greater than a hydrostatic pressure then prevailing at the base of the ship""s ullage tank.
Considering the amount and constituents of gas produced by the inert gas generator, pressured by the compressor, and delivered to the piping to be bubbled upwards through the ballast water, the system preferably serves to render the ballast water hypoxic to xe2x89xa61 ppm oxygen, hypercapnic to xe2x89xa720 ppm carbon dioxide, and acidic to pHxe2x89xa67.
This is achieved at a rate that will, most preferably, permit the entire maximum ballast water of a ship to be treated to these levels in a period less than, most preferably, one-half the normal voyage duration of the ship minus the required time for aquatic nuisance species to die to the 90% level. This is only to say that the shipboard ballast water gaseous infusion system is sized to (i) the task at hand, (ii) the time available for the completion of the task, and (iii) the resilience to die off (from hypercapnia, anoxia and acidic conditions) of the ANS to hand, all at an adequate safety margin. Most typically all the ballast water on a ship will be treated so as to reach desired dissolved gas levels in less than, most preferably, one day, and will be held at those levels for, most preferably, at least two days, and more commonly more than four days. It is, or course, totally acceptable and beneficial to hold the conditions that kill ANS for weeks and longer, should the usage of the ship and its ballast tanks so permit. There is no harm incurred in dumping ballast water having those gas concentrations that are, in accordance with the present invention, different from normal seawater into the sea, where the evacuated ballast water is so quickly diluted that it is not deemed capable of harming even the most delicate marine organisms proximate the release point.
These and other aspects and attributes of the present invention will become increasingly clear upon reference to the following drawings and accompanying specification.